A vehicle with an idling stop function stops the supply of fuel to the engine, when predetermined idling stop conditions are satisfied. Therefore, the engine stalls when the rotational speed of the engine decreases. Subsequently, a motor directly coupled to the engine shaft is triggered to start rotation when the brake pedal is released, for instance, and the engine enters a restart state in which the rotational speed of the engine rises. Then, when predetermined conditions are satisfied, fuel injection is restarted, and furthermore driving of the motor is stopped and the vehicle transfers to a state where the engine rotates due to energy from fuel combustion only. When in an engine restart state, the torque transmitted to the vehicle is controlled by the motor drive force and the energy from fuel combustion. However, if this controllability declines, there is a problem in that racing of the engine and variation in the torque transmitted to the tires occurs, and the feeling for the driver becomes worse.
The difficulty in this problem is that even if the torque of the motor is continuous, the torque output of the engine is intermittent, and the output torque also fluctuates greatly depending on the state of combustion. Consequently, the torque due to combustion which starts up suddenly must be detected, and furthermore, the torque transmitted to the vehicle must be controlled precisely by reducing the torque of the motor, in accordance with magnitude of the torque.
However, in order to control the torque precisely, it is suitable to detect the torque directly. Desirably, it is necessary to detect the torque due to combustion in the engine which displays great fluctuation in the output torque. Therefore, the internal pressure of the cylinders is detected by using a sensor for detecting the internal pressure of the cylinders of the engine, for example, and the engine torque is detected by converting the detected internal pressure of the cylinders into the engine torque. Alternatively, a torque sensor for detecting the engine axle torque is installed in the torque transmission path from the engine to the tires so as to detect the engine torque. Therefore, a sensor of this kind is required, and there is a problem in that the cost of the vehicle rises as a consequence.
Various methods for resolving these problems have been proposed, in which the torque of the engine and motor is controlled without detecting the torque of engine and motor.
For example, in the starting device for an engine disclosed in PTL 1, when the engine is started up, motoring start control is carried out to raise the rotational speed of the engine by motoring, whereupon fuel injection start control is carried out to raise the rotational speed of the engine by torque from the combustion of fuel. In PTL 1, precise control of the torque is not required, since no time period is provided for controlling the torque transmitted to the vehicle.
For example, in the starting device for an engine disclosed in PTL 2, in the time period for controlling the torque transmitted to the vehicle by the drive force of the motor and energy from combustion of fuel, the motor is subjected to rotational speed feedback control based on the differential between a predetermined target rotational speed and the current rotational speed, thereby suppressing variation in the rotational speed of the engine due to increase in the torque from combustion of fuel.